fix: move Renovate preset to repo root for cross-repo compatibility

The // subdirectory syntax doesn't work reliably with the Mend-hosted
Renovate GitHub App on private repos. Moving to default.json in the
repo root uses the standard github>owner/repo reference syntax.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: owine